An incentive for developing starch esters particularly useful in coating compositions is the continued uncertainty over the availability of raw materials derived from petrochemicals. The problem is compounded by the fact that the coating industry, for example, does not represent sufficient outlet for any of the basic petrochemicals to command a stable long-term supply. Moreover, many of the feedstocks for the petrochemical industry, e.g. gas, oil, coal, etc. may be diverted into energy applications particularly at times of high demand. One approach to the problems is to diversify the raw materials supply by relying on agricultural products which are renewed on an annual basis and, if needed, can be expanded to meet the demands by increasing the acreage of crops, e.g. corn, potatoes, etc.
For example, starch is presently being produced at rates exceeding 10 billion pounds per year and is being used for many industrial applications and because of the chemical similarity between starch and cellulose, the latter which has been used in coatings, it is surprising that starch and its derivatives have not received more attention as a potential raw material particularly for the coating industry. It is known that starch in its various natural forms is both water sensitive and brittle and, therefore, must be chemically modified before obtaining the properties necessary for coatings. The highly modified starches, e.g. the type needed in coatings are not commercially available. However, a number of chemically-modified starches, i.e. high molecular weight with low D. S. values showing some promise include the starch esters, starch ethers, starch urea-formaldehyde resins, starch copolymers, etc.
Standard grades of the high molecular weight starches comprise approximately 80% amylopectin having average molecular weights ranging from 1 to 30 million and about 20% of amylose having average molecular weights ranging from about 40,000 to 300,000. The structure of amylopectin and amylose may be illustrated by formulas I and II, wherein n designates the number of linear and branched anhydroglucose repeat units. ##STR1##